A team of researchers says it has taken a fresh look at data from a particular kind of stellar explosion and confirmed what scientists have long believed — that the universe is expanding at an ever-increasing rate. That conclusion is the same one that led scientists in the 1990s to identify a mysterious cosmic force known as dark energy.
The new findings directly contradict a study published earlier this year that claimed the universe’s expansion was no longer speeding up — a conclusion that would have upended our basic understanding of how the cosmos works.
“The universe is still accelerating,” said astrophysicist Brodie Popovic of the University of Southampton in England, one of the lead researchers behind the study, which was published this month in the journal Monthly Notices of the Royal Astronomical Society. “There’s still a lot we don’t know and are excited to learn, but we think we’re on the right track,” Popovic added.
The research team, which included two Nobel Prize recipients, used observations from two separate datasets involving what are known as Type Ia supernovas — powerful stellar explosions — to calculate enormous distances across the cosmos. These explosions occur when an object called a white dwarf, the dense leftover core of a low- to intermediate-mass star at the end of its life, is destroyed.
This category of supernova has become a key tool for astronomers because all such explosions appear to release roughly the same amount of light. That means scientists can use how bright or faint they appear from Earth to figure out how far away they are — essentially using them as cosmic measuring sticks. By tracking the brightness of these events, scientists can determine how fast the universe is expanding and how that rate has changed over time. Because light takes time to travel through space, observing distant objects is effectively a look back into the past.
The universe began with the Big Bang approximately 13.8 billion years ago and has been growing ever since. In 1998, scientists revealed that this growth is actually speeding up, attributing the acceleration to a poorly understood invisible force called dark energy. Ordinary matter — stars, planets, gas, dust, and everything familiar on Earth — makes up an estimated 5% of the universe’s total contents. Dark matter, which is detected through its gravitational pull on galaxies and stars, accounts for roughly 27%, while dark energy is estimated to comprise about 68%.
The earlier 2025 study, also published in the same journal, argued that dark energy is fading and has stopped driving the universe’s accelerating expansion.
“Type Ia supernovae are the premier tool for measuring the expansion history of the universe, and provided the first evidence in 1998 that cosmic expansion is accelerating due to dark energy,” said astrophysicist Adam Riess of Johns Hopkins University in Maryland, a co-author of the new study and a Nobel laureate in physics in 2011 for helping discover the accelerating expansion of the universe.
Riess went on to explain the core disagreement: “Over the past decade, a group at Yonsei University has argued that supernova distances should be calibrated differently by accounting for the ages of the stars that eventually explode, and that this ‘age effect’ could substantially alter the evidence for acceleration. In our study, we found no evidence for the claimed ‘age effect’ in the largest calibrated supernova samples used by the cosmology community over the last decade.”
Astrophysicist Young-Wook Lee of Yonsei University in Seoul, one of the leaders of the earlier 2025 study, stood by his team’s work. Lee said the arguments put forward by the authors of the new study have “serious methodological flaws or lead to conclusions that are internally inconsistent by their own logic.”
The researchers behind the new study said they remain confident in both their methods and their conclusion that the universe’s expansion is indeed still accelerating.
The true nature of dark energy remains one of science’s biggest unsolved mysteries. New tools, including the recently opened Vera Rubin Observatory in Chile and the Nancy Grace Roman Space Telescope — set for launch in August — could shed new light on the question.
“We’re hoping the new data we get from Vera Rubin and the Nancy Grace Roman Space Telescope will help us narrow down what dark energy really is,” Popovic said.
